In the field of nuclear medicine, images of the internal structures or functions of a patient's body are formed by using gamma cameras to detect radiation emitted from within the body after the patient has been injected with a radiopharmaceutical substance. A computer system generally controls the gamma cameras to acquire data and then processes the acquired data to generate the images. Nuclear medicine imaging techniques include Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma ray pairs that are emitted in coincidence in opposite directions due to electron-positron annihilations. PET imaging is therefore often referred to as "coincidence" imaging.
One factor which has a significant impact on image quality in nuclear medicine is non-uniform attenuation. Non-uniform attenuation refers to the attenuation of radiation emitted from an organ of interest before the radiation can be detected. Such attenuation tends to degrade image quality. A technique which has been used to correct for non-uniform attenuation is transmission scanning, in which gamma radiation is transmitted through the patient to a corresponding scintillation detector and used to form a transmission image. The transmission images provide an indication of the amount of attenuation caused by various structures of the body and can therefore be used to correct for attenuation in the emission images.
For purposes of performing attenuation correction on PET images, transmission scans have commonly been implemented using coincidence transmission sources. However, for various reasons it may be desirable to perform a transmission scan for PET using a single-photon ("singles") source. See, e.g., S. K. Yu et al., "Single Photon Transmission Measurements in Positron Emission Tomography Using .sup.137 Cs," Phys. Med. Biol., vol. 40, 1995, and R. A. deKemp, "Attenuation Correction in Positron Emission Tomography Using Single Photon Transmission Measurement," McMaster University, Hamilton, Ontario, Canada, September 1992. Coincidence events generally represent only a small fraction of the total detected events during an imaging session. Consequently, a singles transmission source is preferable because of its higher associated countrate in comparison to a coincidence transmission source. A higher transmission countrate tends to provide a higher signal-to-noise ratio than a lower countrate does. Because of its higher efficiency and the fact that no coincidence is required, an attenuation correction technique which uses a singles transmission source requires a much shorter acquisition time than an attenuation correction technique which uses a coincidence source. In addition, a technique which uses a singles source tends to suffer less deadtime loss than a technique which uses a coincidence source (i.e., from too much activity at the detector nearest to the source in the coincidence case).
For various reasons, it may be desirable to perform the transmission scan after the patient has been injected with the radiopharmaceutical. For example, post-injection transmission scanning reduces the likelihood of patient motion between the transmission scan and the emission scan, which can degrade image quality. Post-injection transmission scanning also reduces the overall scanning time, because it eliminates the waiting period required for the radiopharamaceutical to reach its best uptake (which is typically close to one hour). One problem with post-injection transmission scanning, however, is that it causes emission radiation to be present during the transmission scan, which may be erroneously detected as transmission radiation. In certain systems, it may be possible to use energy discrimination to distinguish between emission activity and transmission activity, i.e., the transmission radiation and the emission radiation can be distinguished by the differences in their energies. However, energy discrimination becomes less effective if the transmission source and the emission source (the radiopharmaceutical) have photopeaks that are relatively close together in energy. For example, it may be desirable to use a Cs.sup.137 transmission source with a photopeak at 662 keV in conjunction with a Flouro Deoxi Glucose (FDG) coincidence emission source with a photopeak at 511 keV. Because, the photopeaks are so close together, some of the emission photons may have energy values that fall within the transmission energy acceptance window. As a result, some of the emission photons may be incorrectly detected as transmission photons, introducing artifacts into the transmission image. This effect is referred to as emission contamination of a transmission scan.
Therefore, it is desirable to have a technique for correcting for emission contamination of a transmission scan in a nuclear camera system. It is further desirable that such a technique take into consideration and correct for spatial variations in the emission activity. In addition, it is desirable that such a technique can be used to correct for emission contamination of a transmission scan which uses a singles transmission source.
Another common problem in nuclear medicine is deadtime loss. Deadtime refers to the inability of a gamma camera detector to distinguish between two radiation-induced scintillation events which occur very close together in time due to the time required to process individual events. Deadtime loss can be defined as the difference between the true countrate ("singles rate") and the observed countrate which results from detector deadtime. In an ideal system in which there is no deadtime loss, the observed countrate equals the true countrate. In contrast, in a system that is subject to deadtime loss, the observed countrate is lower than the true countrate.
One known technique for correcting for deadtime loss is to apply a single, global correction factor, which is applied after the data has been acquired. See, e.g., R. J. Smith et al., "Simultaneous Post Injection Transmission and Emission Contamination Scans in a Volume Imaging PET scanner," 1995 IEEE Nuclear Science Symposium and Medical Imaging Conference Record, vol. 3, pages 1781-85, 1995. The use of a global correction factor, however, has disadvantages. In particular, deadtime loss is dependent upon the singles rate; as the singles rate increases, the deadtime loss also increases. Because the singles rate varies spatially (i.e., depending on projection angle and, if an axially moving transmission source is used, axial position), the deadtime loss is spatially dependent. Therefore, the use of a global deadtime correction factor does not account for the spatial dependency of deadtime losses and may therefore result in inaccuracies in the transmission image. Hence, it would be desirable to have a technique for deadtime correction which takes into consideration the spatial dependencies of deadtime losses. It would be further desirable to have such a technique which also corrects for emission contamination in a transmission scan with the advantageous features discussed above.